Mechanical Reasoning and Spatial Aptitude
What this domain tests
Mechanical Reasoning and Spatial Aptitude checks how well you understand simple machines and how parts move together, and how well you can rotate, fold, or assemble objects in your mind. Expect gears, pulleys, levers, shafts, linkages, and orthographic or net-to-solid questions.
Core rules
- For meshing gears, speed ratio = driver teeth ÷ driven teeth; the driven gear turns in the opposite direction.
- An idler gear reverses direction again but does not change the overall ratio.
- In an open belt drive, pulleys turn the same way; in a crossed belt, opposite ways.
- Lever moment = effort × effort arm = load × load arm at balance. Longer effort arm lowers effort.
- A movable pulley shares load between rope sections; count supporting sections, not just pulleys.
- For spatial items, track one feature at a time: a notch, dot, or shaded face.
Worked example
A 40-tooth gear turning at 120 rpm drives a 20-tooth gear. Find the driven speed and direction. The driver has twice as many teeth, so the driven gear turns twice as fast: 120 × 40 ÷ 20 = 240 rpm. Because the gears mesh externally, it turns in the opposite direction. If an idler were inserted, the final gear would turn in the same direction as the driver, while the 240 rpm magnitude would remain if the same 40-tooth and 20-tooth gears were used.
Traps
- Reversing the gear ratio: put driver teeth on top when finding driven speed.
- Assuming a belt always keeps pulleys turning the same way.
- Counting total pulleys instead of load-supporting rope sections.
- Choosing a rotated view that matches the front but not the hidden edges.